HDL modification by secretory phospholipase A(2) promotes scavenger receptor class B type I interaction and accelerates HDL catabolism.

During inflammatory states plasma levels of high density lipoprotein (HDL) cholesterol and apolipoprotein A-I (apoA-I) are reduced. Secretory group IIa phospholipase A(2) (sPLA(2)) is a cytokine-induced acute-phase enzyme associated with HDL. Transgenic mice overexpressing sPLA(2) have reduced HDL levels. Studies were performed to define the mechanism for the HDL reduction in these mice. HDL isolated from sPLA(2) transgenic mice have a significantly lower phospholipid content and greater triglyceride content. In autologous clearance studies, (125)I-labeled HDL from sPLA(2) transgenic mice was catabolized significantly faster than HDL from control mice (4.24 +/- 1.16 vs. 2.84 +/- 0.1 pools per day, P < 0.008). In both sPLA(2) transgenic and control mice, the cholesteryl ester component of HDL was more rapidly catabolized than the protein component, indicating a selective uptake mechanism. In vitro studies using CHO cells transfected with scavenger receptor class B type I (SR-BI) showed that sPLA(2)-modified HDL was nearly twice as efficient as a substrate for cholesteryl ester transfer. These data were confirmed in in vivo selective uptake experiments using adenoviral vector overexpression of SR-BI. In these studies, increased hepatic selective uptake was associated with increased (125)I-labeled apolipoprotein uptake in the kidney. We conclude that during inflammation sPLA(2) hydrolysis of HDL phospholipids alters the lipid composition of the particle, allowing for more efficient SR-BI-mediated selective cholesteryl ester uptake. This enhanced SR-BI activity generates HDL remnants that are preferentially catabolized in the kidney.

The class B, type I scavenger receptor promotes the selective uptake of high density lipoprotein cholesterol ethers into caveolae.

The uptake of cholesterol esters from high density lipoproteins (HDLs) is characterized by the initial movement of cholesterol esters into a reversible plasma membrane pool. Cholesterol esters are subsequently internalized to a nonreversible pool. Unlike the uptake of cholesterol from low density lipoproteins, cholesterol ester uptake from HDL does not involve the internalization and degradation of the particle and is therefore termed selective. The class B, type I scavenger receptor (SR-BI) has been identified as an HDL receptor and shown to mediate selective cholesterol ester uptake. SR-BI is localized to cholesterol- and sphingomyelin-rich microdomains called caveolae. Caveolae are directly involved in cholesterol trafficking. Therefore, we tested the hypothesis that caveolae are acceptors for HDL-derived cholesterol ether (CE). Our studies demonstrate that in Chinese hamster ovary cells expressing SR-BI, >80% of the plasma membrane associated CE is present in caveolae after 7.5 min of selective cholesterol ether uptake. We also show that excess, unlabeled HDL can extract the radiolabeled CE from caveolae, demonstrating that caveolae constitute a reversible plasma membrane pool of CE. Furthermore, 50% of the caveolae-associated CE can be chased into a nonreversible pool. We conclude that caveolae are acceptors for HDL-derived cholesterol ethers, and that caveolae constitute a reversible, plasma membrane pool of cholesterol ethers.

Scavenger receptor BI mediates the selective uptake of oxidized cholesterol esters by rat liver.

High density lipoprotein (HDL) can protect low density lipoprotein (LDL) against oxidation. Oxidized cholesterol esters from LDL can be transferred to HDL and efficiently and selectively removed from the blood circulation by the liver and adrenal in vivo. In the present study, we investigated whether scavenger receptor BI (SR-BI) is responsible for this process. At 30 min after injection, the selective uptake of oxidized cholesterol esters from HDL for liver and adrenal was 2.3- and 2.6-fold higher, respectively, than for native cholesterol esters, whereas other tissues showed no significant difference. The selective uptake of oxidized cholesterol esters from HDL by isolated liver parenchymal cells could be blocked for 75% by oxidized LDL and for 50% by phosphatidylserine liposomes, both of which are known substrates of SR-BI. In vivo uptake of oxidized cholesterol esters from HDL by parenchymal cells decreased by 64 and 81% when rats were treated with estradiol and a high cholesterol diet, respectively, whereas Kupffer cells showed 660 and 475% increases, respectively. These contrasting changes in oxidized cholesterol ester uptake were accompanied by similar contrasting changes in SR-BI expression of parenchymal and Kupffer cells. The rates of SR-BI-mediated selective uptake of oxidized and native cholesterol esters were analyzed in SR-BI-transfected Chinese hamster ovary cells. SR-BI-mediated selective uptake was 3.4-fold higher for oxidized than for native cholesterol esters (30 min of incubation). It is concluded that in addition to the selective uptake of native cholesterol esters, SR-BI is responsible for the highly efficient selective uptake of oxidized cholesterol esters from HDL and thus forms an essential mediator in the HDL-associated protection system for atherogenic oxidized cholesterol esters.

SR-BII, an isoform of the scavenger receptor BI containing an alternate cytoplasmic tail, mediates lipid transfer between high density lipoprotein and cells.

The scavenger receptor class B, type I (SR-BI), binds high density lipoprotein (HDL) and mediates selective uptake of cholesteryl ester from HDL and HDL-dependent cholesterol efflux from cells. We recently identified a new mRNA variant that differs from the previously characterized form in that the encoded C-terminal cytoplasmic domain is almost completely different. In the present study, we demonstrate that the mRNAs for mouse SR-BI and SR-BII (previously termed SR-BI.2) are the alternatively spliced products of a single gene. The translation products predicted from human, bovine, mouse, hamster, and rat cDNAs exhibit a high degree of sequence similarity within the SR-BII C-terminal domain (62-67% identity when compared with the human sequence), suggesting that this variant is biologically important. SR-BII protein represents approximately 12% of the total immunodetectable SR-BI/II protein in mouse liver. Subcellular fractionation of transfected Chinese hamster ovary cells showed that SR-BII, like SR-BI, is enriched in caveolae, indicating that the altered cytoplasmic tail does not affect targeting of the receptor. SR-BII mediated both selective cellular uptake of cholesteryl ether from HDL as well as HDL-dependent cholesterol efflux from cells, although with approximately 4-fold lower efficiency than SR-BI. In vivo studies using adenoviral vectors showed that SR-BII was relatively less efficient than SR-BI in reducing plasma HDL cholesterol. These studies show that SR-BII, an HDL receptor isoform containing a distinctly different cytoplasmic tail, mediates selective lipid transfer between HDL and cells, but with a lower efficiency than the previously characterized variant.

Alternative forms of the scavenger receptor BI (SR-BI).

The class B, type I scavenger receptor has been implicated as a receptor for high density lipoprotein (HDL). We have isolated a murine cDNA clone encoding an alternative form of SR-BI that differs in the putative cytoplasmic domain of the receptor. This variant form, likely the result of alternative mRNA splicing, is designated SR-BI.2. SR-BI.2 mRNA was detected in mouse tissues known to express SR-BI and tissue-specific differences in the relative abundance of SR-BI.2 were apparent. In mouse adrenal glands, SR-BI.2 represented approximately one-third of total SR-BI mRNA, whereas in mouse testes, SR-BI.2 represented the major mRNA species (79% of total). SR-BI.2 was also detected in the human cell lines examined, namely HeLa, HepG2, and THP-1 cells. CHO cells transfected with the mouse SR-BI.2 cDNA expressed significant levels of SR-BI.2 protein and acquired the ability to take up fluorescent lipid (DiI) from DiI-HDL. Alternative splicing of SR-BI represents a potentially important process for the regulation of SR-BI expression and function.

Compensated coronary microvascular growth in senescent rats with thyroxine-induced cardiac hypertrophy.

We tested the hypothesis that coronary angiogenesis in response to chronic thyroxine (T4) treatment is not limited by age. Male Fischer 344 rats aged either 8 (young adult) or 24 (senescent) mo were studied after receiving either L-thyroxine (0.2 mg/kg sc) or vehicle for 2 mo. Heart weight-to-body weight ratio, compared with age-matched controls, increased by 47 and 44% in 8- and 24-mo T4 groups, respectively. Maximal myocardial perfusion per unit mass, measured in diastole-arrested, maximally dilated, isolated hearts, was similar in T4 rats and their age group controls; however, flow tended to be lower in senescent than in young adult rats. Thus the cross-sectional area of the coronary vessels grew in proportion to the increase in cardiac mass. Morphometric analyses, based on image analysis, showed that capillary length density was slightly lower in the midmyocardium but not the epimyocardium of the 24-mo T4 group compared with their age group controls. However, volume density, surface density, and intercapillary distance were not influenced by T4 treatment and the presence of cardiac hypertrophy. We conclude that in this model of cardiac hypertrophy 1) coronary vessel growth parallels the increase in ventricular mass, 2) capillaries grow by proliferation and an increase in diameter, and 3) vascular growth is not notably compromised during senescence.

Effects of hypothyroidism and hypertension on myocardial perfusion and vascularity in rabbits.

We considered the role of thyroid hormones in the growth of the coronary microvasculature during various levels of afterload. Arterial pressure was increased for 3 mo in hypothyroid and euthyroid rabbits via the Page (1-kidney, 1-wrap) method. Systolic pressures (mmHg +/- SE) indicated that the groups could be characterized as follows: euthyroid sham normotensive (103 +/- 5), euthyroid Page hypertensive (158 +/- 24), hypothyroid Page normotensive (110 +/- 5), and hypothyroid sham hypotensive (87 +/- 5). The hypothyroid groups were characterized by bradycardia. Left ventricular weight-to-body weight ratios were higher in the Page groups than in the controls. Minimal coronary vascular resistance (MCVR) was elevated in the euthyroid Page group but was not affected by hypothyroidism. Lumen diameters of the major resistance vessels tended to be larger in the hypertensive rabbits. Capillary length density was greater in the hypothyroid than in the euthyroid groups. We conclude that 1) the elevated MCVR in the hypertensive rabbits is due mainly to a failure of the major resistance vessels to increase in size or number rather than to a decrease in vascular lumen diameter, and 2) hypothyroidism does not affect maximal myocardial perfusion or lumen diameter of resistance vessels but facilitates capillary growth. The latter may be due to the presence of bradycardia.

Dipyridamole-induced capillary growth in normal and hypertrophic hearts.

Chronic increases in myocardial blood flow have been shown to stimulate capillary proliferation in normal growing hearts. It is unknown, however, if elevated myocardial blood flow stimulates precapillary and/or capillary growth in hearts undergoing hypertrophy. Accordingly, renal hypertension was produced in rabbits (Page, 1-kidney, 1-wrap model) in which one group of Page (n = 9) and one group of normotensive sham (n = 10) rabbits were given dipyridamole (4.0 mg/kg sc) twice daily for 2 mo. Another group of Page (n = 7) and sham (n = 12) rabbits received vehicle injections. In separate acute studies performed on conscious rabbits, this does of dipyridamole increased myocardial blood flow 35-60% over time without altering transmural distribution of flow or systemic blood pressure. Two months later, minimal coronary vascular resistance (MCVR/100 g) was calculated from perfusion during maximal coronary vasodilation in conscious animals. Histomorphometric methods were then utilized to evaluate various indexes of capillarity in perfuse-fixed hearts. Systolic pressure and left ventricle weight-to-body weight ratios were significantly higher in Page vs. sham rabbits; dipyridamole treatment did not alter these parameters within either group. Similarly, dipyridamole treatment did not significantly alter MCVR/100 g values in either normotensive or hypertensive rabbits. In contrast, dipyridamole treatment increased endomyocardial capillary length density by 33% in the hypertensive group (P less than 0.05) and 11% in the sham group (P not significant) compared with the respective vehicle-treated rabbits. In addition, intercapillary distance was significantly reduced in the endomyocardial region of both groups receiving dipyridamole injections.(ABSTRACT TRUNCATED AT 250 WORDS)

Sympathectomy stimulates capillary but not precapillary growth in hypertrophic hearts.

Sympathetic nerves are known to influence vascular growth, but their role in coronary vascular adaptations to pressure-overload left ventricular (LV) hypertrophy is unknown. Accordingly, regional sympathectomy (SYMX) was produced by painting a ring of phenol on the posterior third of the LV in seven renal hypertensive (Page: 1 kidney, 1 wrap) and seven normotensive (sham: 1 kidney, no wrap) rabbits. Two months later, maximal myocardial blood flow (MBF) following dipyridamole-induced coronary vasodilation was determined with microspheres in the intact anterior and the sympathectomized posterior regions of conscious rabbits. Histomorphometric methods were then utilized to evaluate capillary density (CD), intercapillary distance (ICD), and volume density (VD) of subepicardial and endocardial samples of each region of perfused-fixed hearts. The Page procedure significantly increased systolic blood pressure (+29%) and LV wt/body wt (+20%) above sham rabbits. In both sham and Page groups, MBF was not significantly different between intact and sympathectomized regions within either group. SYMX did not significantly alter CD, ICD, or VD between regions in the sham animals. In contrast, SYMX significantly increased CD (+30%) and VD (+26%) and decreased ICD (-21%) in the subendocardial region of Page animals. Regional SYMX did not alter myocyte cross-sectional area in Page animals. We conclude that SYMX neither 1) significantly increases resistance vessel cross-sectional lumen area in either normal or hypertrophic hearts, nor 2) significantly influences capillary growth in normal hearts, but SYMX does 3) promote capillary growth in hearts undergoing hypertrophy in response to hypertension.